Avoiding flashover in MV switchgear

Corona is a serious issue in metal clad switchgear because of its highly destructive nature and is the cause of most flashovers in medium voltage metal clad electrical equipment. The fundamental cause of this symptom is an electrical breakdown of air brought on by electrical stress and poor air quality within the switchgear. If not prevented, a flashover can cause enormous damage and injure personnel.

Tests have shown that when a flashover occurs the temperature is in excess of 20 000°C, with a bulk of the damage being caused by secondary explosions from a build up of compressed unburned gases within the enclosure.

Dealing with corona discharge in the case of metal clad switchgear is unpredictable and caution must be used at all times. With today’s instruments used for condition monitoring and predictive maintenance, we are able to help combat and verify this problem and do it successfully.

All experienced electrical maintenance personnel are familiar with the various surprises when entering any electrical equipment. Always expect the unexpected, they say. So prior to opening any equipment a visual inspection, ultrasound detection and sometimes infra-red (IR) imaging is necessary.

Fig. 1a: Insulation decay on a 5 kV feeder. Fig. 1b: Air flow in the dust during corona activity.

Corona discharges create byproducts, viz., nitric acid and carbon which cause a continuous decay of insulation to take place, which makes the situation highly unpredictable. With corona being the cause of most flashovers in metal clad switchgear one can predict that a flashover is imminent in all cases.

There are various signs which can identify corona, some with no indication of corona using nondestructive testing, but with closer visual inspection we find that corona discharge had been present, but is not active as shown in Fig. 1.

In both these images, flashover was imminent if the problem had not been rectified. Even though the corona was dormant at the time of inspection, it would have started discharging again with the appropriate relative air density, temperature, moisture and lack of air movement. Fig. 1a shows insulation decay phase to phase on a 5 kV feeder. Fig. 1b shows actual air flow in the dust during corona activity on a conductor.

In this case the deterioration is on the conductor insulation and corrosion on the metal cladding. In other cases we find the corona discharge extremely active. This is when extra caution is taken because a flashover could occur at any time. Corona also produces airborne ultrasound, ultraviolet (UV) light and if advanced into arcing it will sometimes show a thermal signature.

Fig. 2: Report sheet showing corona discharge.

This is a perfect opportunity for ultrasonic detection, UV/corona imaging, IR/thermal imaging to find out where and why the corona is discharging as shown in Fig. 2. Here a thermogram was able to show a temperature rise on the 15 kV buss insulators, verified by the UV corona images.

The problem in this case is decay in the horizontal insulators causing electrical stress to initiate the corona. This all started with contamination entering the switchgear, trapping moisture which then, under the right conditions, started producing corona.

Then with nitric acid, UV light, ozone and carbon deteriorating the insulation the corona eventually spreads on its own. Corona is extremely serious and should be dealt with and eliminated as soon as possible.

We can stop the corona with our cleaning equipment. Sometimes permanent and sometimes temporarily, depending on the situation. At times when entering equipment the environment changes inside the metal clad compartment when a door is opened or a cover is removed.

This, on occasion, stops the corona discharge which makes it a little tougher to verify. Another byproduct of corona is ozone and the odour can be identified. Also there may be a chance of a thermal signature as shown in Fig. 3. These thermograms represent two different cases of defective stress cones. Fig. 3a shows a 5 kV shielded cable with a stress cone and Fig. 3b shows a 15 kV shielded cable with stress cone.

The temperature rise is minimal, but the problems are very serious. Both stress cones were developing corona underneath the insulation and will eventually flash to ground.

As can be seen from Fig. 3 the temperature rise is less than 2°C. Some corona discharge events sometimes show up well in a thermal image as shown in Fig 4.

Active corona in Fig 4 shows a 4 kV junction box which is very close to flashover on three phases and had to be rebuilt because the corona damage was so extensive. It was possible to temporarily stop the activity with cleaning which kept it energised until the utility had a scheduled shutdown.

Corona is not always easy to find, but with the use of UV/corona imaging, IR/thermal imaging, ultrasonic detection and visual inspection it is possible to verify your findings, which means it is getting easier.

Case study

Fig. 5 shows a 5 kV main feeder for an underground mine. No outage was needed for this corona problem.

After we cleaned the junction box and separated the leads, a heater was installed to keep the moisture out. Corona is an electrical breakdown of air phenomenon that results from the ionising of gasses due to a high electrical stress. This stress is often caused by the reasons mentioned earlier, i.e. contamination and poor insulation values.

Fig. 5: Main 5 kV feeder for an underground mine.

After continually working in live metal clad electrical equipment, Highvec has come to take the corona situation very seriously. By wearing fire retardant clothing, knowing our surroundings and exits at all times and using proper tools, we have been able to extinguish many corona problems, benefiting everyone’s safety and insuring the integrity of the equipment to prevent costly outages. There are times when corona is quite obvious, without using any test equipment. Again we use extreme caution when investigating the cause.

Fig. 6a shows severe corona in a 15 kV interrupter switch. The green material on the busbar is corrosion from active corona and the black material on the bakelite insulation is carbon residue from active corona and arcing. This particular switch was very close to flashover.

Fig. 6b shows what Fig. 6a was like after it was cleaned. We were able to stop the corona activity temporarily, to buy time for a scheduled power outage. The problem in this case was the red bakelite divider was tight against the 15 kV busbar and was starting to short out.

The problem was resolved by cutting the insulating divider a 5 cm back from the bus. There is no particular tool which will verify all corona problems all the time because the corona discharge is erratic and unpredictable, especially in the early stages. Ultrasound detection can be used to verify if air-born ultrasound is present from the corona discharge.

An IR camera can be used to identify temperature rise from corona discharge. A UV/corona camera is the ultimate tool for verifying corona discharge. If the discharge is dormant, then none of this equipment will indicate or verify anything.

A visual inspection may indicate there was discharge previously, which will tell you that it will return. When tracking down corona we have found that it does not always leave a thermal signature. Corona creates little or no heat, but as it progresses into arcing and tracking a thermal signature can sometimes be found.

Fig. 7: Heat coming into the visual spectrum.

Corona is most intense in the 300 to 400 nm range in the UV spectrum. When it exceeds the 400 nm range corona can be seen in the visual spectrum if there is no interference from background light. This is similar to heat coming into the visual spectrum at approximately 585°C at the 700 nm level, as shown in Fig. 7.

We performed a small corona experiment using the three different spectrums. In Fig. 8 we show two different scenarios of active corona at 5 kV. In the left and right columns are the three different spectrums all at the same time.

At the top we have the UV spectrum using the corona camera and corona is evident. In the middle nothing can be seen in the visual spectrum. At the bottom in the IR spectrum, using thermal imaging, there is no thermal signature where the corona activity is present.

Fig. 8: Two different scenarios of active corona at 5 kV.

Conclusion

With the use of infrared imaging, corona imaging and ultrasound detection, it is possible to find problem areas before they become a health and safety risk along with serious financial burdens. Cleaning and maintaining electrical equipment under live conditions has proven to be very beneficial to everyone.

Acknowledgement

This article was published in the February/March 2017 edition of Transmission and Distribution and is republished here with permission.